doc: update CONTRIBUTING.md about the changelog structure

Signed-off-by: Etienne Marais <[email protected]>

Author: maiste

CONTRIBUTING.md

@@ -36,12 +36,15 @@ Our development process is as follows:
   Exchanges in meetings and chat are valuable, but we insist on deliberating and
   recording the what and why of our work in the persistent record of GitHub.
 - Changes are submitted via GitHub pull requests against the `main` branch.
+- Technical changes must appear in the project Changelog. Learn more on how to
+  [update the changelog][update changelog].
 
 If you are looking to get started, check our issues tagged with [good first
 issue][good first issue] or [help wanted][help wanted].
 
 [good first issue]: https://github.com/ocaml/dune/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22good%20first%20issue%22
 [help wanted]: https://github.com/ocaml/dune/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22help%20wanted%22
+[update changelog]: https://github.com/ocaml/dune/blob/main/CONTRIBUTING.md#updating-the-changelog
 
 Developer documentation
 -----------------------
@@ -131,3 +134,16 @@ you have the rights to submit this work under the same open source license.
 [js]: https://www.janestreet.com/
 [tarides]: https://tarides.com/
 [hack]: ./doc/hacking.rst
+
+Updating the Changelog
+----------------------
+
+In Dune, the Changelog is in `CHANGES.md`. As they are many contributions at
+the same time and to prevent an infinite cycle of rebase, the `CHANGES.md` file
+is only updated when releasing Dune. To keep track of the changes, they must be
+recorded inside `doc/changes/`. The name of the file is `<PR number>.md`.
+Depending on the nature of the change, it can live inside three categories:
+`added`, `changed`, `fixed`, where:
+- `added` records every new feature or behaviour inside of Dune
+- `changed` records a difference of behaviour with the previous version
+- `fixed` records any change that restores the behaviour as expected